WHAT IS BIOGAS

What is Biogas

Biogas typically refers to a gas mixture produced by the anaerobic degradation of organic matter. It is mainly composed of methane and carbon dioxide and other trace gases (H2S, H2O,..). Biogas can be produced from different natural or man-made substrates that disintegrate by bacterial processes. Being a renewable carbon neutral energy source biogas is used to generate electricity, heat or transportation fuel.

Composition

Biogas consists mainly of methane and carbon dioxide, and traces of hydrogen sulphide, nitrogen and in some cases of siloxanes.

The energy content of biogas is determined by the proportion of methane. The biogas composition depends primarily on the design and operational parameters of the anaerobic digestion process and on the sources.

Feedstock

Biogas can be produced from a large range of feedstocks suitable for anaerobic digestion. Potential feedstocks are biomass or waste substrates in the form of solids, slurries and liquids arising from the agricultural, industrial or public sector.

Agricultural waste

Liquid and solid manure

Sludge from waste water treatment plants

Municipal waste (Biowaste, food leftovers)

Energy crops

The biogas yields and the methane content from different organic matters are reported in Figure 1.

Production

Biogas is produced by anaerobic digestion. In this biological process the organic matter is decomposed by bacteria in the absence of oxygen.

The main steps of the anaerobic digestion process are:

Hydrolysis: Break-down of the organic matter by micro-organisms into soluble organic compounds

Acido-&Acetogenesis: Conversion of the decomposed matter into organic acids

Methanogenesis: Conversion of the organic acids into a methane-rich gas (i.e. biogas)

The left over indigestible material and dead micro-organisms containing valuable plant nutrients like nitrogen and potassium is referred to as digestate. The digestate can be used as a substitute to chemical based fertilisers and soil conditioner.

The anaerobic process producing biogas takes place in a digester. The digester is covered or encapsulated to capture the biogas for flaring, heat and power generation or for feeding a natural gas network.

Advantages

Biogas has many advantages, especially with regard to sustainable development and environmental protection:

Renewable resource and mostly local resource:

Readily available and sustainable

Independent supply

Carbon neutral when combusted (Not contributing to the greenhouse effect)

Application

Biogas is combusted or oxidized in gas engines, combined heat and power (CHP) engines or fuel cells to generate renewable energy.

The main applications are:

Electricity generation

Heat generation

Space heating

Hot water

Cold generation (Trigeneration: cold, heat and power)

Transportation fuel (compressed natural gas)

The typical global efficiency of a CHP is 85-90%, with an electrical efficiency of 28-47% and a thermal efficiency of34-55%.

Facts & Potentials

An average biogas plant with an installed capacity of 190kWel can supply 450 homes with electricity and about 30 homes with heat. This avoids, on average, 650 tonnes of CO2 and substitutes 20,000 kilograms of mineral fertilizer.

Regional development

The development of biogas as energy source varies greatly in Europe (Figure 2). Germany represents with around 50% of the total amount of biogas produced in Europe, the biggest producer. In Luxembourg, the biogas production increased by a factor 8 between 2002 and 2013, reaching 15,2 ktoil eq. of biogas in 2013.

Figure 2: Evolution of biogas production (expressed in oil equivalents) in different European countries